with positive parameters \(\nu \)  and \(\ell \) , where \(K_{\nu }\)  is a modified Bessel function, \(\Gamma ()\)  is the Gamma function and \(\Vert  \cdot \Vert \)  denotes the Euclidean distance. The parameter \(\nu \)  (which we set to \(3/2\) ) controls the differentiability of the sample paths of the GP and is fixed, whereas \(\ell \)  is a lengthscale parameter that controls the rate of change of the sample paths in \(\mathcal {T}\)  and is a hyperparameter. The GP hyperparameter set \(\Omega = \lbrace \ell , \sigma _\epsilon ^2\rbrace \)  is estimated via a maximum likelihood estimation (MLE) procedure frequently followed in fitting GP models [1]}, [2]}.
